Use of variable speed AC Drives for speed control and Energy saving in Pulp & Paper
Abstract
This paper describes use of "VARIABLE SPEED AC DRIVES" for Energy Saving by controlling speed of squirrel Cage Induction motor for applications like Fans, Pumps & Compressors in Pulp & Paper Industry.
Introduction
Energy Conservation Act, 2003 has identified Pulp & Paper Industry as one of the key industries which need to implement energy savings, with an energy saving potential as high as 15%. Pumps and fans are among the major consuming applications in the paper making process. They are used to transport liquid/air by establishing necessary draught. The volume of liquid/air needs to be varied as per the requirement of process. Conventionally, a louver damper or throttling valve is provided in the duct. The Pump/fan is always run at rated speed and volume of liquid/air is controlled by increasing or decreasing the damper/throttling valve opening. As the damper/throttling valve opening decreases, pressure drop across the same increases, resulting in loss of some amount of energy across it.
The volume of air/gas/liquid can also be varied by changing the speed of Pump/fan drive which will avoid damper/throttling valve losses. Speed of a Cage Induction Motor can be varied by varying the supply frequency to the motor using a variable speed drive.
Energy saving by speed control:
Fig. 1 shows typical performance characteristics of a centrifugal Pump/fan (i) at rated speed & (ii) at reduced speed. Fig. 1 also shows typical system curves (comprising of duct, fan etc.) (A) with damper control & (B) without damper control. As air volume increases, the resistance caused by increasing turbulence & friction in the fan/duct system results in rapid increase in the pressure that must be developed by the Pump/fan. The pressure is proportional to square of the flow (volume).
The point of intersection of the system curve & the Pump/fan characteristics is the operating point. With damper control employed, system will operate at point "A" where volume requirement is "Q1" and corresponding pressure developed in the duct will be "p1". Then power requirement of the Pump/fan at operating point "A" will be "P1".
p1 * Q1
P1 = -------------*10-3 KW
?F
Where ?F = Pump/fan efficiency
Similarly, power requirement for operating point "B" for same volume "Q1" with Pump/fan running at reduced speed i.e. with damper fully open will be "P2".
p2 * Q1
P2 = -------------*10-3 KW
?F
Therefore power saving achieved by running the Pump/
fan at lower speed without damper control is equal to
"(P1-P2)" kW and is approximately equal to the area
of rectangle " p1ABp2" (refer fig. 1).
Net power saving for the system is equal to
( P1 - P2) * Q1
P3 = ----------------- *10-3 KW
?F * ?M
Where ?M = Motor efficiency
Total energy saved per annum when you run the Pump/ fan at lower speed is equal to
P3 * H kWHr
Where H = Annual run time of Pump/fan in hours
In most of the cases, need for running the Pump/fan at lower speed arises because of following reasons.
Requirement to run the process at lower than designed capacities
Cumulative safety margins built during various design stages like duct design, Pump/fan design etc.
Hence there is always a good potential for energy saving.
Estimation of energy saving.
As an example, typical case is discussed here for estimating energy saving
Relation between various system parameters
p ? Q2 where p = Pressure, Q = Flow || Q ? N N = Motor Speed || p0 ? p*Q p0 = Motor Output Power
Case Study
Motor Rating = 360 kW || Application = Pump
Sample Calculation
Rated Parameters:
Rated flow: 231 M3/HR || Discharge Pressure: 37.27kg/cm2 || Suction Pressure : 2.87 kg/cm2 || Differential Pressure: 34.4 kg/cm2 || Pump efficiency: 72% || Present Running Parameters: (As measured) || Flow: 280 MT/HR (Specific Gravity: 1.37) i.e. 204 M3/HR || Suction Pressure: 2.4 kg/cm2 || Discharge Pressure: 42.5kg/cm2 || Differential Pressure: 40.1kg/cm2 || Valve Opening: 24.1% || Pump efficiency at 204 M3/HR: 68% || Motor efficiency (assumed): 90%
Input Power to the Motor, P= (40.1*9.81*10)*(204/3600)/ (0.68*0.90)= 364kW
(The effect of static head is not considered, as it will be same for the next case with variable speed drive)
Ratio (Actual Flow / Rated Flow) = 204/231= 0.8847
If the pump speed is reduced by the same proportion,using variable speed AC Drive, the differential pressurewill reduce by square of the same ratio. Hence, newdifferential pressure will be =34.4* (204/231) 2 = 26.92 kg/cm2
Revised Input Power to the Motor P' = (26.92*9.81*10)*(204/3600)/(0.68*0.90)= 244 kW Net Savings = 364-244= 120 kW If VSD losses are = 10 kW ( 96% efficiency), Net Power saved is 110 kW With Unit Power rate of Rs 4.00, Total savings per year will be 110*8760*4.0 = Rs 38.5 L
Note: It is assumed that the monitoring points of suction & discharge pressures are such that the difference in these two pressures eliminates the effect of static pressure. However, if actual static head is given, the calculations can be refined further. Similarly, resistive drop in the piping is neglected.
Key considerations for selection of inverter
A. Temperature :-
The AC Drives are designed to operate at certain ambient condition and they need to be derated if operated at higher ambient temperature. Further, the design temperature should be "Site Maximum Ambient Temperature plus additional minimum 5o C" to take care of temperature rise inside the panel.
B. Overload Requirement in AC Drives :-
Overload i.e. to deliver more than its capacity for a small duration is always desired to overcome known & unknown loading conditions in the process. Specify the overload as the application demands.
C. Harmonics :-
Like any other switching device, AC Drives is one of the culprit of generating harmonics. Every AC Drive addition will lead to increase in net Total Harmonic Distortion of the plant.
Possible solutions to reduce harmonics generated by AC Drives are :
Install a DC or AC input reactor. This is must irrespective of ratings.
Go for 12 pulse or higher input rectification configuration. The harmonics generated by this system will be of the order of 12n 1. Thus, most predominant harmonics like 5th, 7th, 17th, 19th …… , are eliminated on the HV side of transformer. Thus, 12-pulse rectification configuration drastically reduces harmonics current. Hence for higher kW ratings 12-pulse rectification scheme helps in addressing the problem of harmonics.
D. Is your AC Motor, General Purpose or Inverter Grade ???
Most of us use general-purpose motors with inverters. When motor to inverter panel cable distances are long (in excess of 30m), due to standing wave phenomenon, high voltage spikes are generated at motor terminals. General-purpose motors are not designed for such voltage spikes and results into premature burn out / failure of their winding. To safe guard motor against this voltage spike, appropriate protection device needs to be provided at output of the inverter e.g. output reactor, sinusoidal filter.
Thus it is very important to install output reactor or sinusoidal filter at the output of inverter.
Conclusion:
Use of Variable Speed Drive for Pump/fan results in significant energy saving. Other benefits, which accrue, are precise speed control, less wear & tear of mechanical system etc.
- Chirag Shah